This invention concerns the characterization of porous permeable media, for example wood objects attacked by parasites.
It is known, from document U.S. Pat. No. 5,357,063 for example, that acoustic energy can be used to detect and identify objects buried in the ground. Acoustic energy is injected into the soil and a reflected signal is received and processed to produce a image which is representative of the reflected energy. This technique, which is applied in particular for detection of buried objects, is useful for not very porous media. If the medium examined is too porous, there are problems of propagation of the acoustic energy which affects the accuracy of the method.
U.S. Pat. No. 5,285,688 also involves an acoustic method. It discloses a system for detection of wood-destroying insects, in particular a system which detects acoustic emissions produced by these insects and which reveals their presence. This system is based on listening to active insects and consequently requires the presence of living insects. This is not always the case for media to be characterized.
A porous permeable medium may also be characterized by methods involving impregnating a medium with a dense liquid, such as mercury, or with water. These methods require the presence of a liquid. They are thus more particularly applicable to areas where a liquid is already present in the media in question, for example in the field of drilling (see patents U.S. Pat. No. 5,309,098 and U.S. Pat. No. 5,610,522). In other areas of application such as those involving works of art, there is a risk of deterioration of the medium to be characterized.
To avoid the presence of a liquid, NMR imaging using the resonance of helium 3 has been used to visualize for example the porosity of lungs. This technique was disclosed in the following articles:
xe2x80x9cVoir les poumons grace a l""heliumxe2x80x9d (See the lungs using helium) by M. Leduc and E. Otten, in the review La Recherche no 287, May 1996, pages 41-43;
xe2x80x9cLow-field 3He nuclear magnetic resonance in human lungsxe2x80x9d by L. Darrasse, G. Guillot, P. J. Nacher and G. Tastevin, C.R. Acad. Sci. Paris, t. 324, series IIb, p. 691-700, 1997.
It involves having a patient inhale polarized helium 3 and visualizing the gas inhaled by nuclear magnetic resonance. In this technique, high resolution imagery must be done to measure the porosity from the measurement of the pore dimensions on the images, thus requiring complex and costly experimental means.
To overcome the drawbacks of the prior art, a method for characterization is proposed using NMR of polarized gases, such as 3He or 129Xe. The invention thus involves a method for characterizing a porous permeable object including the following steps:
placing the object in an air-tight container,
creating a partial vacuum in the container containing the aforesaid object,
measuring the residual pressure in the container in which the partial vacuum is formed,
introducing a quantity of polarized gas in the container in partial vacuum,
measuring the pressure in the container after introduction of the aforesaid quantity of polarized gas,
determination of the density of the polarized gas introduced into the container from the aforesaid pressure measurements,
NMR measurement of the mass quantity of polarized gas contained in a known volume of the object placed in the air-tight container,
determination of the porosity of the object by calculation of the ratio of the mass quantity of polarized gas to the density of the polarized gas introduced into the container.
The object can be placed in a rigid container. It could also be a container sufficiently flexible so that the depressurizing provokes adherence of the container to the object. The quantity of polarized gas introduced in the container may then be such that the container continues to adhere to the object.
The polarized gas could be 3He or 129Xe.
The NMR measurement can be done by a NMR area system. This measurement can be done by a NMR area system involving CMPG sequences, thus allowing for determination of medium porosity dimensions.
The NMR measurement can also be done by a NMR volume system.
The invention also involves a device for characterizing a porous permeable object including:
an air-tight container apt to receive the aforesaid object,
means for measurement of the pressure in the air-tight container,
means for depressurizing the aforesaid air-tight container,
means for introducing a quantity of polarized gas into the depressurized air-tight container,
means for NMR measurement of the mass quantity of polarized gas contained in a known volume of the object placed in the air-tight container,
means for determination of the density of the polarized gas introduced in the container from data supplied by the means of measurement and determination of the porosity of the object from measurement of the mass quantity of the polarized gas and the density of the polarized gas introduced into the container.